Air-stepping in the neonatal mouse: a powerful tool for analyzing early stages of rhythmic limb movement development.

There is a lack of experimental methods in genetically tractable mouse models to analyze the developmental period at which newborns mature weight-bearing locomotion. To overcome this deficit, we introduce methods to study L-DOPA induced air-stepping in mice at postnatal day 7 (P7) and P10. Air-stepping is a stereotypic rhythmic behavior that resembles mouse walking overground locomotion, but without constraints imposed by weight-bearing, postural adjustments or sensory feedback. We propose that air-stepping represents the functional organization of early spinal circuits coordinating limb movements. After subcutaneous injection of L-DOPA (0.5 mg/g), we recorded air-stepping movements in all four limbs and electromyographic (EMG) activity from ankle flexor (tibialis anterior, TA) and extensor (lateral gastrocnemius, LG) muscles. Using DeepLabCut pose estimation we analyzed rhythmicity and limb coordination. We demonstrate steady rhythmic stepping of similar duration from P7 to P10, but with some fine-tuning of interlimb coordination with age. Hindlimb joints undergo a greater range of flexion at older ages indicating maturation of flexion-extension cycles as the animal starts to walk. Electromyographic recording of TA and LG show alternation, but with more focused activation particularly in the LG from P7 to P10. We discuss similarities with neonatal rat L-DOPA induced air-stepping and infant assisted walking. We conclude that limb coordination and muscle activations recorded with this method represent basic spinal cord circuitry for limb control in neonates and paves the way for future investigations on the development of rhythmic limb control in genetic or disease models with correctly or erroneously developing motor circuitry.